Fix uidvalidity_valid check.

Summary:
It appears that server UIDVALIDITY values sometimes *decrease*.  Per RFC3501
the server is supposed to *increase* the UIDVALIDITY if UIDS are actually
invalid. So it should in theory be safe to ignore a decremented UIDVALIDITY? In
practice, that appears to be the case for the actually affected accounts.

Anyways, flip the comparison. Fixes T471, sort of.

Test Plan: Stare at code; RFC.

Reviewers: spang

Reviewed By: spang

Maniphest Tasks: T471

Differential Revision: https://review.inboxapp.com/D583
Unify invalid credential handling across event/contact syncs.

Summary:
One of the reasons our error count is so high is that we try to sync contacts
for invalid accounts every five minutes and log a bunch of errors. To be
consistent with events sync, simply exit the sync loop in such cases.

Test Plan: Unit test added.

Reviewers: charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D588